Contribution docs don't mention docs.sh

[ChrisChinchilla]

The contribution guide doesn't mention that scripts/docs.sh exists or that it needs to pass for docs related PRs to pass. It's fairly similar to make html but not quite as it breaks a build.

[chriseth]

It is actually even stricter: All source code snippets in the documentation have to compile without warnings. This is tested inside test/cmdlineTests.sh which we could perhaps extract into its own script.

[ChrisChinchilla]

@chriseth And this is a very good thing, I just want to make sure future contributors know that.

I am very much a supporter of automating docs testing, I'll show you my language listing stuff some day :)